The present invention relates to a method of making ultrafine WCxe2x80x94Co alloys from a well dispersed mixture of fine and non-agglomerated WC and Co powders, optimised grain growth refiner additions and carbon content using low temperature sinter/sinter-HIP conditions.
It is well known that decreasing the WC grain size confers performance advantages to cemented carbide in many applications e.g. PCB (Printed Circuit Board) machining, wood machining, metal cutting. Maintaining a submicron WC grain size requires the use of grain growth refiners such as VC, Cr3C2, TaC etc. The finer the WC grain the greater the necessary addition of said refiners. In some applications e.g. metal cutting fineness of the WC grain size should not greatly reduce toughness, otherwise edge life will suffer. Grain refiners may reduce toughness if used in excessive amounts.
In other applications e.g. PCB machining, fineness of WC grain size is of paramount importance with toughness demand being secondary. Commercially available ultra fine cemented carbide grades already use a grain size of about 0.4 xcexcm. But to reduce the WC grain sizes to below 0.4 xcexcm requires novel raw material and processing technique.
DE 40 00 223 (Mitsubishi) discloses a cemented carbide based on WC with 6-14 wt-% binder phase containing vanadium and chromium whereby the ratio Cr/(Cr+V) is  less than 0.95 and  greater than 0.50. U.S. Pat. No. 4,539,041 discloses the making of metallic powders by a process for reducing oxides, hydroxides or metal salts with the aid of polyols. Particularly, when starting with cobalt hydroxide it is possible to obtain powders of metallic cobalt as essentially spherical, non-agglomerated particles. Such Co powder is herein referred as polyol cobalt.
U.S. Pat. No. 5,441,693 discloses a method of making cemented carbide with an extremely uniform structure by using Co-powder produced according to the above mentioned polyol method and with submicron grain size.
It is an object of the present invention to provide a method of making cemented carbide with WC grain size less than 0.8 xcexcm and with a low content of grain refiners.
In one aspect, the present invention provides a method of making a cemented carbide having a submicron WC grain size, the method comprising the steps of: (i) forming a powder mixture comprising deagglomerated submicron WC produced by carbothermal reaction, Co powder having deagglomerated spherical grains of submicron size with a narrow grain size distribution wherein at least 80% of the grains have sizes in the interval x+0.2x with the interval of variation of 0.4x is not smaller than 0.1 xcexcm, a carbon content of approximately the amount necessary to provide eta phase formation, and  less than 1 wt. % grain growth inhibitor; (ii) milling the powder mixture; and (iii) sintering.
According to the method of the present invention cemented carbide compositions with extremely fine microstructure, average grain size  less than 0.8 xcexcm, essentially no grains larger than 1.5 xcexcm, suitable for toughness demanding machining operations are made by milling deagglomerated submicron WC powder produced by carbothermal reaction with a cobalt powder having deagglomerated spherical grains of about 0.4 xcexcm average grain size and with a narrow grain size distribution wherein at least 80% of the particles have sizes in the interval xxc2x10.2x provided that the interval of variation (that is 0.4x) is not smaller than 0.1 xcexcm. Preferably the cobalt powder is polyol cobalt. If the carbon content of the powder mixture to be sintered is held close to etaphase formation no, or a relatively low amount ( less than 1 wt-%) of grain growth refiners such as VC and Cr3C2 need to be added. Sintering with HIP takes place at relatively low temperatures (i.e.,  less than 1400xc2x0 C.). The sintered cemented carbide has a Co-content of 70-85% in terms of cobalt magnetic measurements assuming pure cobalt.
In a preferred method the average WC grain size is further reduced to below 0.4 xcexcm by using an optimum VC+Cr3C2 addition in which the ratio of VC/Cr3C2 in wt-% is 0.33-1.0, preferably 0.5-0.9, most preferably 0.7-0.8 for PCB-applications and 0-0.5 for metal cutting, preferably 0-0.2 for non ferrous machining and 0 for ferrous machining. Preferably sintering is performed using gas pressure sintering also referred to as sinter-HIP.
In a first embodiment particularly useful for finish and general machining of non-ferrous materials the cemented carbide has 6-10% Co, 0.0-0.3 VC, 0.3-0.75 Cr3C2 and rest WC less than 0.8 xcexcm.
In a second embodiment particularly useful for rough machining in demanding work materials (e.g. austenitic stainless steels) the cemented carbide has 10-16% Co, 0.5-1.2 Cr3C2 and rest WC less than 0.8 xcexcm.
In a third embodiment particularly useful for very tough machining operations, or those with very low cutting speed, (e.g. broaching) the cemented carbide has 16-20% Co, 0.8-1.8 Cr3C2 and rest WC less than 0.8 xcexcm.
In a fourth embodiment particularly useful for PCB and non-metallic routing and slot drilling, the cemented carbide has 5-8% Co, 0.1-0.6 VC, 0.25-0.6 Cr3C2 and rest WC less than 0.4 xcexcm.
In a fifth embodiment particularly useful for PCB micro drilling the cemented carbide has 8-12% Co, 0.2-0.9 VC, 0.4-0.9 Cr3C2 and rest WC less than 0.4 xcexcm.
In a sixth embodiment particularly useful for wood-machining of solid wood or fibreboard the cemented carbide has 2-5% Co, 0.05-0.2% VC, 0.1-0.25% Cr3C2 and rest WC less than 0.4 xcexcm.